Sound from Chevy Suburban seaker suddenly quit

BMORRIS

2004 CHEVROLET SUBURBAN

V8

4WD

AUTOMATIC

88,000 MILES

I have a 2004 Chevy Suburban K1500. The speakers just stopped making sounds (AM, FM, XM, CD, Onstar) while driving on the interstate. All lighted indicators, channel displays and onstar "phone" displays are still working. Although no sound comes from the speakers, if I plug a headset into the rear cabin console I can hear the sound.
I checked the amplifier, radio and several other affiliated fuses and they appear OK. (Three months ago the local dealer replaced passenger side airconditioning system and had to remove/ reinstall the dash but the same mechanics indicate that should not have contributed to the problem) Looking for a recommended troubleshooting manual that covers this area or advice from someone who has had the problem. Thank you